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Geodynamics Series

 

Keywords

  • Geology, Structural
  • Orogeny

Article

GEODYNAMICS SERIES, VOL. 19, PP. 39-56, 1987

Early Paleozoic back-arc deformation in the Lachlan fold belt, southeastern Australia: Implications for terrane translations in eastern Gondwanaland

C. L. Fergusson

Much of the eastern part of the Lachlan Fold Belt in southeastern Australia consists of the Benambra terrane which in the Ordovician contained an eastward facing island arc with a back-arc basin to the west. Part of the back-arc basin is preserved in the Tabberabbera Belt of the southwestern Benambra terrane. The southern Tabberabbera Belt is dominated by east–west trending folds that formed in a Silurian back-arc deformation and is divided by a fault (the Wonnangatta Line) into two major domains. The southwestern domain is dominated by shallowly plunging steeply inclined folds with wavelengths of up to 10 km. These folds gradually tighten towards the north where an early recumbent fold is rooted in a zone of tectonic melange up to 2 km in width at the domain boundary. The horizontal enveloping surfaces and the low amplitude-to-wavelength ratios of regional folds are consistent with a thin-skinned tectonic style and fold vergence indicates tectonic transport towards the south. The northeastern domain consists of upright close to isoclinal folds with large amplitude-to-wavelength ratios reflecting thick-skinned deformation. There is no overall sense of tectonic transport and younger structures have caused significant multiple deformation.
The Silurian back-arc deformation developed due to Chilean-style oblique-slip subduction along the convergent boundary east of the Ordovician island arc. Regional dextral shear caused the east–west folding and also formed several major northwest-trending strike-slip faults that caused southwest translation of the Benambra terrane. Structural contrasts across the Tabberabbera Belt reflect southwesterly thickening pre-Ordovician crust that probably formed as the western passive continental margin of the back-arc basin. The tectonic pattern is consistent with strike-slip translation and convergence along the Early Paleozoic eastern Gondwanaland margin. There is little evidence for accretion of major allochthonous oceanic terranes as has occurred in western North America.

Citation: Fergusson, C. L. (1987), Early Paleozoic back-arc deformation in the Lachlan fold belt, southeastern Australia: Implications for terrane translations in eastern Gondwanaland, in Terrane Accretion and Orogenic Belts, Geodyn. Ser., vol. 19, edited by E. C. Leitch and E. Scheibner, pp. 39–56, AGU, Washington, D. C., doi:10.1029/GD019p0039.

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